In recent years, the trend has been toward drilling deeper gas wells with higher pressure and as a result, demand has risen for a joint for oil and gas well pipes having performance properties superior to those employing API threads (round, buttress) and having high tensile joint strength and excellent gas-leak-tightness as well as resistance to corrosion and erosion. Among the desired properties of such joints, gas-leak-tightness is especially important. This is because the leakage of an inflammable gas might lead to fire or explosion while the leakage of a poisonous gas such as hydrogen sulfide might cause a disastrous accident.
As conventional ways to enhance the gas-leak-tightness of a threaded portion, it is known to make the clearance as small as possible by strictly controlling the machining tolerance, to plate the threaded portion with a soft metal such as Sn having high malleability, and to apply to the threaded portion a compound grease mixed with a very fine metallic powder and a mineral oil thickened with a calcium stearate or a lithium stearate. Also, it is known to be important to control the torque and number of turns at the time of make-up. However, even with these means, the leakage of gas cannot be perfectly prevented under the conditions of high temperature and high pressure currently being encountered in gas and oil wells. Because of this, special types of joints have been used to improve gas-leak-tightness. Those of one type have a seal portion which employs metal-to-metal contact between a pin (steel pipe) and a box (coupling) forming the joint, while those of another type have an annular groove with a Teflon ring inserted therein.
Several special joints of the former type are known. For example, in one of such joints peripheral shoulder having a convex conical inner face is provided as the inner end portion of the box, a non-threaded inner face extending between the end of the threaded portion and said peripheral edge shoulder is provided in correspondence thereto, the end of the pin is formed as a concave conical inner face and a non-threaded outer face is provided between the threaded portion and the end of the pin (Japanese Patent Publication No. 18096/1970). This joint is characterized in that the concave conical face of the male thread and the convex conical face of the female thread abut to effect metal-to-metal contact.
On the other hand, as a special joint similar to said joint, there is known one wherein a peripheral shoulder having a convex conical inner face is provided on the inner end portion of the box and a non-threaded slant face is formed on the tip of the inner thread of the box to give the pin a shape corresponding to that of the box. In this joint the first seal portion is formed by concave and convex conical shoulder faces similar to the above mentioned joint. However, there is the difference that the expanded portions are provided on the pin and the slant face of the box to form the second seal portion by means of metal-to-metal contact between these portions (Laid-Open Patent Application No. 17125/1973).
In the special joint based on the metal-to-metal contact for assuring gas-leak-tightness, such effects as are described in the respective specifications can be expected. However, these joints suffer the following problems:
First, in the former case, the tip of the pin is the only sealing spot. Therefore, in order to obtain effective metal-to-metal contact, it is necessary to make up for this by using a large torque sufficient to compensate for the clearance caused by machining error. However, excessive make-up is apt to cause uneven plastic deformation at the contact portion, as will be explained later. Therefore, initial gas-leak-tight effect is apt to be considerably decreased at the time of repeat make-up, and, moreover, galling cracks are likely to occur on the thread portion. This is a critical problem since repeat make-up is effected up to three times on casings and up to ten times on tubings, and in either case, it is, of course, essential that no cracks occur and high gas-leak-tightness be assured.
In the actual use environment of a well pipe joint, the gas-leak-tightness under tension, compression and bending load etc. also become an important factor. For example, when internal pressure and tensile load act on the joint, there is a danger that the interfacial pressure of the sealing portion effecting metal-to-metal contact may decrease with the increase in tensile load, finally causing minute clearance between the gas-leak-tight faces. Also, since the precision-machined sealing face on the tip of the pin is completely exposed, it is apt to be marred in the field regardless of how much care is taken. When the sealing face is damaged, it is difficult to maintain the gas-leak-tightness of the metal-to-metal contact.
On the other hand, the joint described in said Laid-Open Patent Application No. 17125/1973 has higher contact face pressure on the expanded face of the non-threaded slant face provided on the pin and the box, so that even if the make-up is effected with considerably lower torque than that used with the joint described in said Patent Publication No. 18096/1970, a comparable gas-leak-tightness can still be obtained. However, there is the disadvantage that even though a higher interfacial pressure is obtained, the metal-to-metal contact made by the expanded portion is nearly linear, meaning that high machining accuracy is required in the contact portion. Therefore, clearance at any part of the circumference will cause leakage. Moreover, if excessive load is imposed, the line contact position will shift, causing a decrease in face pressure and lowering gas-leak-tightness.